Monday, November 23, 2015

New research slated for publication in Physical Review B shows that “cold spots” can be localized within a molecule, leaving single atoms with temperatures near absolute zero, while other parts of the molecule rest around a comparatively balmy 100 Kelvin.

Ordinarily, temperature is a measure of the collective kinetic energy of a multitude of molecules, so talking about it on the quantum scale becomes a bit hairy—instead of having to do with the energy of the molecules themselves, sub-molecular temperature has to do with the kinetic energy of electrons as they move around the nuclei of the molecules’ atoms.

One end of a pyrene molecule reaches
temperatures near absolute zero, while
other parts are nearly 100 Kelvin warmer.Image Credit: Abhay Shastry, University of Arizona

When a molecule of pyrene, the four-ringed carbon structure pictured at right, is heated under specific conditions, the infrared light that carries the heat energy forms standing waves in the pyrene’s electron cloud. Based on the molecule’s geometry, these waves add together in some places and cancel out in others, leading to high-energy and low-energy spots, respectively.

If this sounds at all familiar, it might be because you’ve encountered this same phenomenon in the kitchen! In a microwave oven, which uses radiation with a wavelength of several centimeters, constructive and destructive interference cause similar hot spots and cold spots, leading to tongue burns for countless hungry and impatient souls. (This is why cooking instructions tell you to let your food stand a minute after microwaving—it gives the heat time to diffuse through the food from the hot spots to the cold ones.) Where those spots end up depends on the shape and size of the oven and its radiation source, as well as the wavelength of the light used, but the interference nodes can be found experimentally by placing chocolate or marshmallows in a microwave without a rotating turntable; the parts of the chocolate in regions of constructive interference will melt much faster than those in the canceled-out regions. Since infrared light has a wavelength much shorter than that of microwaves, it can create these “hot spots” on a much smaller scale, leading to inhomogeneities in the temperature profile of the pyrene. This result is just the latest in a series of publications from the University of Arizona, exploring heat transfer at the nano-scale. Already, similar work from U of A has shown unusual thermal flows in the single-atom-thick lattice of graphene, and this newest paper promises to shed more light on how those behaviors emerge.

2 comments:

Something seems off about your explanation. The distance between hot spots in a microwave is on the order of a wavelength. But a pyrene molecule (~1 nm) is much much smaller than the wavelength of infrared light (>1 um) (by a factor of 1000 or more).

The explanation I think uses a good analogy. The temperature variations actually depend upon the relative transmissions from the hot and cold reservoirs. The wavelength here roughly correlates with a variation in the local density of states. So, if one could get the waves from the hot reservoir to destructively interfere-- you will get cold spots.